Glasses for flat panel displays

ABSTRACT

Glasses are disclosed which are used to produce substrates in flat panel display devices. The glasses exhibit a density less than about 2.45 gm/cm 3  and a liquidus viscosity greater than about 200,000 poises, the glass consisting essentially of the following composition, expressed in terms of mol percent on an oxide basis: 65-75 SiO 2 , 7-13 Al 2 O 3 , 5-15 B 2 O 3 , 0-3 MgO, 5-15 CaO, 0-5 SrO, and essentially free of BaO. The glasses also exhibit a strain point exceeding 650° C.

CROSS-REFERENCED TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 10/960,594filed on Oct. 7, 2004; which is a continuation of U.S. Pat. No.6,831,029, which is a continuation of U.S. Pat. No. 6,319,867. Thecontents of which are relied upon and incorporated herein by referencein their entirety and the benefit of priority under 35 U.S.C. § 120 ishereby claimed.

FIELD OF THE INVENTION

The present invention relates to alkali-free, aluminosilicate glassesexhibiting desirable physical and chemical properties for substrates inflat panel display devices.

BACKGROUND OF THE INVENTION

Displays may be broadly classified into one of two types: emissive(e.g., CRTs and plasma display panels (PDPs)) or non-emissive. Thislatter family, to which liquid crystal displays (LCDs) belong, reliesupon an external light source, with the display only serving as a lightmodulator. In the case of liquid crystal displays, this external lightsource may be either ambient light (used in reflective displays) or adedicated light source (such as found in direct view displays).

Liquid crystal displays rely upon three inherent features of the liquidcrystal (LC) material to modulate light. The first is the ability of theLC to cause the optical rotation of polarized light. Second, is theability of the LC to establish this rotation by mechanical orientationof the liquid crystal. The third feature is the ability of the liquidcrystal to undergo this mechanical orientation by the application of anexternal electric field.

In the construction of a simple, twisted nematic (TN) liquid crystaldisplay, two substrates surround a layer of liquid crystal material. Ina display type known as Normally White, the application of alignmentlayers on the inner surface of the substrates creates a 900 spiral ofthe liquid crystal director. This means that the polarization oflinearly polarized light entering one face of the liquid crystal cellwill be rotated 90° by the liquid crystal material. Polarization films,oriented 900 to each other, are placed on the outer surfaces of thesubstrates.

Light, upon entering the first polarization film, becomes linearlypolarized. Traversing the liquid crystal cell, the polarization of thislight is rotated 90° and is allowed to exit through the secondpolarization film. Application of an electric field across the liquidcrystal layer aligns the liquid crystal directors with the field,interrupting its ability to rotate light. Linearly polarized lightpassing through this cell does not have its polarization rotated andhence is blocked by the second polarization film. Thus, in the simplestsense, the liquid crystal material becomes a light valve, whose abilityto allow or block light transmission is controlled by the application ofan electric field.

The above description pertains to the operation of a single pixel in aliquid crystal display. High information type displays require theassembly of several million of these pixels, which are referred to assub pixels, into a matrix format. Addressing, or applying an electricfield to, all of these sub pixels while maximizing addressing speed andminimizing cross-talk presents several challenges. One of the preferredways to address sub pixels is by controlling the electric field with athin film transistor located at each sub pixel, which forms the basis ofactive matrix liquid crystal display devices (AMLCDs).

The manufacturing of these displays is extremely complex, and theproperties of the substrate glass are extremely important. First andforemost, the glass substrates used in the production of AMLCD devicesneed have their physical dimensions tightly controlled. The downdrawsheet or fusion process, described in U.S. Pat. Nos. 3,338,696(Dockerty) and 3,682,609 (Dockerty), is one of the few capable ofdelivering such product without requiring costly post forming finishingoperations, such as lapping and polishing. Unfortunately, the fusionprocess places rather severe restrictions on the glass properties,requiring relatively high liquidus viscosities, preferably greater than200,000 poises.

Typically, the two substrates that comprise the display are manufacturedseparately. One, the color filter plate, has a series of red, blue,green, and black organic dyes deposited on it. Each of these primarycolors must correspond precisely with the pixel electrode area of thecompanion, active, plate. To remove the influence of differences betweenthe ambient thermal conditions encountered during the manufacture of thetwo plates, it is desirable to use glass substrates whose dimensions areindependent of thermal condition (i.e., glasses with lower coefficientsof thermal expansion). However, this property needs to be balanced bythe generation of stresses between deposited films and the substratesthat arise due to expansion mismatch. It is estimated that an optimalcoefficient of thermal expansion is in the range of 28−33×10⁻⁷/° C.

The active plate, so called because it contains the active, thin filmtransistors, is manufactured using typical semiconductor type processes.These include sputtering, CVD, photolithography, and etching. It ishighly desirable that the glass be unchanged during these processes.Thus, the glass needs to demonstrate both thermal and chemicalstability.

Thermal stability (also known as thermal compaction or shrinkage) isdependent upon both the inherent viscous nature of a particular glasscomposition (as indicated by its strain point) and the thermal historyof the glass sheet as determined by the manufacturing process. U.S. Pat.No. 5,374,595, disclosed that glass with a strain point in excess of650° C. and with the thermal history of the fusion process will haveacceptable thermal stability for active plates based both on a-Si thinfilm transistors (TFTs) and super low temperature p-Si TFTs. Highertemperature processing (such as required by low temperature p-Si TFTs)may require the addition of an annealing step to the glass substrate toensure thermal stability.

Chemical stability implies a resistance to attack of the various etchantsolutions used in the manufacture processes. Of particular interest is aresistance to attack from the dry etching conditions used to etch thesilicon layer. To benchmark the dry etch conditions, a substrate sampleis exposed to an etchant solution known as 110 BHF. This test consistsof immersing a sample of glass in a solution of 1 volume of 50 wt. % HFand 10 volumes 40 wt. % NH₄F at 30° C. for 5 minutes. The sample isgraded on weight loss and appearance.

In addition to these requirements, AMLCD manufacturers are finding thatboth demand for larger display sizes and the economics of scale aredriving them to process larger sized pieces of glass. Current industrystandards are Gen III (550 mm×650 mm) and Gen III.5 (600 mm×720 mm), butfuture efforts are geared toward Gen IV (1 m×1 m) sizes, and potentiallylarger sizes. This raises several concerns. First and foremost is simplythe weight of the glass. The 50+% increase in glass weight in going fromGen III.5 to Gen IV has significant implications for the robotichandlers used to ferry the glass into and through process stations. Inaddition, elastic sag, which is dependent upon glass density and Young'sModulus, becomes more of an issue with larger sheet sizes impacting theability to load, retrieve, and space the glass in the cassettes used totransport the glass between process stations.

Accordingly, it would be desirable to provide a glass composition fordisplay devices having a low density to alleviate difficultiesassociated with larger sheet size, preferably less than 2.45 g/cm³ and aliquidus viscosity greater than about 200,000 poises. In addition, itwould be desirable for the glass to have thermal expansion between about28-35×10⁻⁷/° C., and preferably between about 28-33×10⁻⁷/° C., over thetemperature range of 0-300° C. Furthermore, it would be advantageous forthe glass to have a strain point greater than 650° C., and for the glassto be resistant to attack from etchant solutions.

SUMMARY OF THE INVENTION

The present invention is founded in the discovery of glasses exhibitingdensities less than 2.45 g/cm³ and a liquidus viscosity (defined as theviscosity of the glass at the liquidus temperature) greater than about200,000 poises, preferably greater than about 400,000 poises, morepreferably greater than about 600,000 poises, and most preferablygreater than about 800,000 poises. Additionally, the glasses of thepresent invention exhibit linear coefficients of thermal expansion overthe temperature range of 0-300° C. between about 28-35×10⁻⁷/° C., andpreferably between about 28-33×10⁻⁷/° C., and strain points higher thanabout 650° C. The glass of the present invention has a meltingtemperature less than about 1700° C. In addition, the glass exhibits aweight loss of less than about 0.5 mg/cm² after immersion in a solutionof 1 part HF 50 wt. % and 10 parts 40% wt. % NH₄F for 5 minutes at 30°C.

The glass of the present invention has a composition consistingessentially of the following composition as calculated in mole percenton an oxide basis: 65-75 SiO₂, 7-13 Al₂O₃, 5-15 B₂O₃, 0-3 MgO, 5-15 CaO,0-5 SrO, and essentially free of BaO. More preferably, the glass of thepresent invention has a composition consisting essentially of thefollowing composition as calculated in mole percent on an oxide basis:67-73 SiO₂, 8-11.5 Al₂O₃, 8-12 B₂O₃, 0-1 MgO, 5.5-11 CaO, and 0-5 SrO.

We have discovered that for glasses having the compositions and physicalproperties discussed above, especially the preferred compositions andpreferred properties, the liquidus viscosity of the glass is stronglyinfluenced by the ratio of the sum of alkaline earths, RO (R=Mg, Ca, Sr)to alumina on a mol % basis, or RO/Al₂O₃=(MgO+CaO+SrO)/Al₂O₃. This ratiois referred to as RO/Al₂O₃, and should be held in the range 0.9 to 1.2.Most preferably, this range should be 0.92<RO/Al₂O₃<0.96 to obtain thehighest liquidus viscosity.

The glasses of the present invention are essentially free of BaO, whichmeans that the glasses preferably contain less than about 0.1 mol % BaO.The glasses of the invention are also essentially free of alkali metaloxides, which means that the glasses preferably contain a total of lessthan about 0.1 mol % of alkali metal oxides. Additionally, these glassesmay contain fining agents (such as the oxides of arsenic, antimony,cerium, tin, and/or the halides, chlorine/fluorine).

In another aspect of the invention, the glasses have a meltingtemperature less than about 1700° C. The glasses of the presentinvention also exhibit a weight loss of less than 0.5 mg/cm² afterimmersion in a solution of 1 part 50 wt. % HF and 10 parts 40% wt. %NH₄F for 5 minutes at 30° C. The glasses are useful as a substrate forflat panel displays. Substrates made from the glass of the presentinvention have an average surface roughness as measured by atomic forcemicroscopy of less than about 0.5 nm and an average internal stress asmeasured by optical retardation of less than about 150 psi.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is concerned with improved glasses for use as flatpanel display substrates. In particular, the glasses meet the variousproperty requirements of such substrates.

The preferred glasses in accordance with the present invention exhibit adensity of less than about 2.45 gm/cm³, preferably less than about 2.40gm/cm³, a CTE over the temperature range of 0-300° C. between about28-35×10⁻⁷/° C., preferably between about 28-33×10⁻⁷/° C., and strainpoints higher than about 650° C., preferably greater than 660° C. A highstrain point is desirable to help prevent panel distortion due tocompaction/shrinkage during subsequent thermal processing.

For more demanding manufacturing conditions such as the fusion process,described in U.S. Pat. Nos. 3,338,696 (Dockerty) and 3,682,609(Dockerty), a glass having a high liquidus viscosity is required.Therefore, in a preferred embodiment of the present invention, theglasses should exhibit a density less than about 2.45 gm/cm³ and aliquidus viscosity greater than about 200,000 poises, preferably greaterthan about 400,000 poises, more preferably greater than about 600,000poises, and most preferably greater than about 800,000 poises. Althoughsubstrates made from glass of the present invention can be made usingother manufacturing processes such as the float process, the fusionprocess is preferred for several reasons. First, glass substrates madefrom the fusion process do not require polishing. Current glasssubstrate polishing is capable of producing glass substrates having anaverage surface roughness greater than about 0.5 nm (Ra), as measured byatomic force microscopy. Glass substrates produced according to thepresent invention and using the fusion process have an average surfaceroughness as measured by atomic force microscopy of less than 0.5 nm.

Chemical durability involves a resistance to attack of the variousetchant solutions used in the manufacture processes. Of particularinterest is a resistance to attack from the dry etching conditions usedto etch the silicon layer. One benchmark of the dry etch conditions isexposure to a etchant solution known as 110BHF. This test consists ofimmersing a sample of glass in a solution of 1 volume of 50 wt. % HF and10 volumes 40 wt. % NH₄F at 30° C. for 5 minutes. Chemical resistance isdetermined by measuring weight loss in terms of mg/cm². This property islisted in Table I as “110 BHF.”

The glasses of the present invention include 65-75 mol %, preferably67-73 mol %, SiO₂ as the primary glass former. Increasing silicaimproves the liquidus viscosity and reduces the density and CTE of theglass, but excessive silica is detrimental to the melting temperatures.The glasses also comprise 7-13 mol %, preferably 8-11.5 mol %, Al₂O₃.Increases in Al₂O₃ content increase glass durability and decrease CTE,but liquidus temperatures increase. At least 8 mol % is required to havethe most desired strain point; however, more than 11.5 mol % results ina less than desired liquidus temperature.

The glasses further include 5-15 mol %, preferably 8-12 mol % boricoxide. Boric oxide lowers the liquidus temperature and density andpreferably is present in at least 8 mol %; however, more than 12 mol %boric oxide will negatively impact the glass strain point.

MgO is present in the glasses of the present invention in an amount of0-3 mol %, preferably 0-1 mol %. Increasing MgO decreases liquidusviscosity, and therefore, no more than 3 mol % MgO should be present inthe glass. However, smaller amounts of MgO may be beneficial forreducing density.

CaO is useful to lower both the melting and liquidus temperatures of theglass; however, more than 11 mol % will result in a less than desiredstrain point and a higher than desired coefficient of thermal expansion.Therefore, the glasses of the present invention can include 5-15 mol %Ca0, but preferably include 5.5-11 mol % CaO.

The RO/Al₂O₃ ratio is in the range 0.9-1.2. Within this range localminima in liquidus temperature can be found, corresponding to cotecticsand/or eutectics in the base system CaO—Al₂O₃—SiO₂. The viscosity curvesof the glasses in this series do not vary significantly, thus thesechanges in liquidus temperatures are the primary drivers for increasesin liquidus viscosity.

Because of their negative impact on thin film transistor (TFT)performance, alkalis such as lithia, soda or potash are excluded fromthe present glass compositions. Likewise, heavier alkaline earth metals(SrO and BaO) will be either minimized or excluded because of theirnegative impact on the glass density. Accordingly, the glasses of thepresent invention may include 0-5 mol % SrO. However, the glasses of theinvention are essentially free of BaO. As used herein, essentially freeof BaO means that the composition contains less than 0.1 mol % of BaO inthe composition.

Fining agents such as As₂O₃, Sb₂O₃, CeO₂, SnO₂, Cl, F, SO₂, etc. mayalso be present to aid in the removal of seeds from the glass. The glassmay also contain contaminants as typically found in commerciallyprepared glasses. In addition, the following oxides can be added at alevel not exceeding 1 mol % without pushing properties outside of theranges described above: TiO₂, ZnO, ZrO₂, Y₂O₃, La₂O₃. Table I listsexamples of glasses of the present invention in terms of mol % alongwith their physical properties. The comparative example in Table 1 isCorning Incorporated's 1737 glass.

The invention is further illustrated by the following examples, whichare meant to be illustrative, and not in any way limiting, to theclaimed invention. TABLE I sets forth exemplary glass compositions inmol percent, as calculated on an oxide basis from the glass batches.These example glasses were prepared by melting 1,000-25,000 gram batchesof each glass composition at a temperature and time to result in arelatively homogeneous glass composition, e.g. at a temperature of about1625° C. for a period of about 4-16 hours in platinum crucibles. Alsoset forth are relevant glass properties for each glass composition,determined on the glasses in accordance with techniques conventional inthe glass art. Thus, the linear coefficient of thermal expansion (CTE)over the temperature range 0-300° C. is expressed in terms of ×10⁻⁷/°C., the softening point (Soft. Pt.), and the annealing point (Ann. Pt.),and strain point (Str. Pt.) are expressed in terms of ° C. These weredetermined from fiber elongation techniques (ASTM references E228-85,C338, and C336, respectively). The density (Den.), in terms of g/cm³,was measured via the Archimedes method (ASTM C693).

The 200 poise temperature (Melt. Temp., ° C.) (defined as thetemperature at which the glass melt demonstrates a viscosity of 200poises [20 Pa·s]) was calculated employing the Fulcher equation fit tothe high temperature viscosity data (measure via rotating cylindersviscometry, ASTM C965-81). The liquidus temperature (Liq. Temp.) of theglass was measured using the standard liquidus method. This involvesplacing crushed glass particles in a platinum boat, placing the boat ina furnace having a region of gradient temperatures, heating the boat inan appropriate temperature region for 24 hours, and determining by meansof microscopic examination the highest temperature at which crystalsappear in the interior of the glass. The liquidus viscosity (Liq. Visc.,in poises) was determined from this temperature and the coefficients ofthe Fulcher equation.

Table I records a number of glass compositions, expressed in terms ofparts by mole on the oxide basis, illustrating the compositionalparameters of the present invention. Inasmuch as the sum of theindividual constituents totals or very closely approximates 100, for allpractical purposes the reported values may be deemed to represent molepercent. The actual batch ingredients may comprise any materials, eitheroxides, or other compounds, which, when melted together with the otherbatch components, will be converted into the desired oxide in the properproportions. For example, SrCO₃ and CaCO₃ can provide the source of SrOand CaO, respectively.

Glasses having the compositions and properties shown in Examples 14 and19 are currently regarded as representing the best mode of theinvention, that is, as providing the best combination of properties forthe purposes of the invention at this time.

Although the invention has been described in detail for the purpose ofillustration, it is understood that such detail is solely for thatpurpose and variations can be made therein by those skilled in the artwithout departing from the spirit and scope of the invention which isdefined by the following claims.

TABLE 1 Composition Batched (mol %) 1 2 3 4 5 6 7 8 SiO₂ 69.75 69.7869.75 69.8 70.7 70.57 69.1 70.85 Al₂O₃ 10.2 10.2 10.3 10.3 10 9.77 10.359.5 B₂O₃ 9.7 9.7 9.5 9.5 10 9.95 10.25 10.1 MgO 0.8 0.8 0.8 0.8 0.120.15 1.25 CaO 7 7 8.5 8.5 9 9.17 9.8 6.7 SrO 2.2 2.2 0.8 0.8 1.3 BaOAs₂O₃ 0.3 0.3 0.4 0.33 Sb₂O₃ 0.3 0.3 0.3 0.1 CeO₂ 0.2 Y₂O₃ SnO₂ 0.050.02 0.05 0.05 0.02 0.02 Cl 0.2 0 0.2 0.2 0.2 RO/Al₂O₃ 0.98 0.98 0.980.98 0.90 0.95 0.96 0.98 CTE (0-300° C., ×l0⁻⁷/° C.) 32.7 31.2 31.9 30.830.5 30.2 31.5 31.4 Dens. (g/cm³) 2.416 2.404 2.394 2.38 2.37 2.3552.375 2.367 Str. Pt (° C.) 672 674 673 679 671 666 673 664 Ann. Pt. (°C.) 727 731 729 734 728 729 729 720 Soft. Pt. (° C.) 984 991 991 9951001 1001 976 992 110 BHF (mg/cm²) 0.15 0.14 0.145 0.135 0.14 0.15 0.180.27 Liq. Temp. (° C.) 1125 1120 1120 1125 1150 1135 1115 1140 Liq.Visc. (p) 5.04E+05 7.06E+05 6.34E+05 6.30E+05 3.66E+05 5.95E+05 5.30E+054.77E+05 Melt. Temp. (° C.) 1659 1668 1650 1659 1668 1680 1635 1686Composition Batched (mol %) 9 10 11 12 13 14 15 16 17 SiO₂ 69.1 69.6568.75 69.1 68.8 69.33 69.65 70.05 69.33 Al₂O₃ 10.35 10.1 10.55 10.210.35 10.55 10.2 9.9 10.55 B₂O₃ 10.25 10.25 10.25 10.55 10.55 9.97 10.2510.25 9.97 MgO 0.15 0.15 0.15 0.15 0.15 0.18 0.15 0.15 O.18 CaO 9.3 9.459.9 9.65 9.8 9.08 8.85 9.25 9.58 SrO 0.5 0.5 0.5 BaO As₂O₃ 0.33 0.4 0.40.33 0.33 0.37 0.4 0.4 0.37 Sb₂O₃ CeO₂ Y₂O₃ SnO₂ 0.02 0.02 0.02 0.020.02 0.02 0.02 0.02 0.02 Cl RO/Al₂O₃ 0.96 0.95 0.95 0.96 0.96 0.93 0.930.95 0.93 CTE (0-300° C., ×l0⁻⁷/° C.) 32.2 31.4 31.8 32 31.8 31.5 30.930.8 31.4 Dens. (g/cm³) 2.377 2.366 2.371 2.364 2.366 2.376 2.369 2.3572.366 Str. Pt (° C.) 671 669 674 666 667 676 671 668 674 Ann. Pt. (° C.)727 725 729 722 723 731 728 726 730 Soft. Pt. (° C.) 983 985 986 980 980985 987 992 987 110 BHF (mg/cm²) 0.15 0.12 0.12 Liq. Temp. (° C.) 10951090 1100 1090 1115 1095 1100 1100 1120 Liq. Visc. (p) 1.05E+06 1.23E+069.19E+05 1.07E+06 5.74E+05 1.19E+06 1.04E+06 1.12E+06 6.23E+05 Melt.Temp. (° C.) 1649 1654 1650 1642 1650 1651 1657 1674 1654 CompositionBatched (mol %) 18 19 20 21 22 23 24 25 26 SiO₂ 69.45 69.33 68.3 70.4570 70.4 70.1 70 68.3 Al₂O₃ 11 10.55 10.9 9.85 10.35 10.05 10 10.23 10.9B₂O₃ 9 9.97 10.4 9.55 9.5 9.69 10.15 9.75 10.4 MgO 0.18 1.1 0.8 0.820.81 0.8 CaO 10 9.08 9.1 5.75 8.5 7.14 7.11 7 9.1 SrO 0.5 1 2.9 0.851.89 1.88 2.22 1.0 BaO As₂O₃ 0.4 0.3 Sb₂O₃ 0.3 0.37 0.3 CeO₂ 0.2 0.2 0.20.2 Y₂O₃ SnO₂ 0.05 0.02 0.05 0.05 Cl 0.2 0.2 RO/Al₂O₃ 0.91 0.93 0.930.99 0.98 0.98 0.98 0.98 0.93 CTE (0-300° C., ×l0⁻⁷/° C.) 32 31.9 34.833.4 32.8 33.5 32.9 33.4 34.5 Dens. (g/cm³) 2.386 2.383 2.403 2.4052.378 2.378 2.374 2.391 2.392 Str. Pt (° C.) 681 675 669 670 672 670 664671 677 Ann. Pt. (° C.) 737 730 725 727 728 727 720 727 732 Soft. Pt. (°C.) 993 984 978 996 988 989 984 989 979 110 BHF (mg/cm²) 0.16 0.3 0.390.18 0.17 0.19 0.19 Liq. Temp. (° C.) 1150 1100 1140 1165 1160 1140 11501150 1150 Liq. Visc. (p) 2.90E+05 1.06E+06 2.40E+05 2.88E+05 2.49E+054.29E+05 2.80E+05 3.11E+05 2.14E+05 Melt. Temp. (° C.) 1642 1655 16171682 1656 1668 1653 1656 1620 Composition Batched (mol %) 27 28 29 30 3132 Comp Ex SiO₂ 68 70 70.4 69.8 71 72 67.6 Al₂O₃ 10.5 10 10.2 10.6 9.99.33 11.4 B₂O₃ 11 10 9.1 9 10 10 8.5 MgO 1.3 CaO 10.5 9 5.68 7.14 9.18.66 5.2 SrO 4.55 3.42 1.3 BaO 4.3 As₂O₃ 0.4 Sb₂O₃ 0.3 0.3 0.3 0.3 0.30.3 CeO₂ Y₂O₃ SnO₂ Cl RO/Al₂O₃ 1.00 0.90 1.00 1.00 0.92 0.93 1.03 CTE(0-300° C., ×l0⁻⁷/° C.) 34.9 32.0 34.4 34.4 32.1 31.2 37.8 Dens. (g/cm³)2.378 2.355 2.420 2.406 2.353 2.342 2.54 Str. Pt (° C.) 659 673 669 672669 672 666 Ann. Pt. (° C.) 713 731 727 729 726 730 721 Soft. Pt. (° C.)967 999 987 984 998 1012 975 110 BHF (mg/cm²) 0.23 0.18 0.2 Liq. Temp.(° C.) 1120 1130 1130 1135 1120 1125 1050 Liq. Visc. (p) 3.07E+055.99E+05 5.67E+05 4.38E+05 8.46E+05 8.79E+05 2.97E+06 Melt. Temp. (° C.)1611 1670 1671 1657 1685 1703 1636

1. An aluminosilicate glass comprising in mol percent on an oxide basis:65-75 SiO₂, 7-13 Al₂O₃, and 5-15 B₂O₃, wherein: (a) said glass has aRO/Al₂O₃ ratio between 0.9 and 1.2, wherein R represents Mg, Ca, Sr, andBa; (b) the glass has a CaO concentration between 5 mol percent and 15mol percent on an oxide basis; (c) the glass has a MgO concentrationbetween 0 mol percent and 3 mol percent on an oxide basis; and (d) theglass has a density less than about 2.45 gram/cm³, a liquidus viscositygreater than about 200,000 poises, and a linear coefficient of thermalexpansion over the temperature range from 0° C. to 300° C. between28×10⁻⁷/° C. and 35×10⁻⁷/° C.
 2. A glass according to claim 1 whereinthe glass comprises in mol percent on an oxide basis: 67-73 SiO₂, 8-11.5Al₂O₃, 8-12 B₂O₃, and 5.5-11 CaO.
 3. A glass according to claim 1wherein the RO/Al₂O₃ ratio is between 0.92 and 0.96, wherein Rrepresents Mg, Ca, Sr, and Ba.
 4. A glass according to claim 1 whereinthe glass has a strain point greater than about 650° C.
 5. A glassaccording to claim 1 wherein the glass has a strain point greater thanabout 660° C.
 6. A glass according to claim 1 wherein the glass has amelting temperature less than about 1700° C.
 7. A glass according toclaim 1 wherein the glass exhibits a weight loss of less than 0.5 mg/cm²after immersion in a solution of 1 part 50 wt. % HF and 10 parts 40 wt.% NH₄F for 5 minutes at 30° C.
 8. A glass according to claim 1 whereinthe glass has a liquidus viscosity greater than about 400,000 poises. 9.A glass according to claim 1 wherein the glass has a liquidus viscositygreater than about 600,000 poises.
 10. A glass according to claim 1wherein the glass has a liquidus viscosity greater than about 800,000poises.
 11. A glass according to claim 1 wherein the glass has a densityless than about 2.40 gram/cm³.
 12. A glass according to claim 1 whereinthe glass has a linear coefficient of thermal expansion over thetemperature range from 0° C. to 300° C. between 28×10⁻⁷/° C. and33×10⁻⁷/° C.
 13. A glass according to claim 1 wherein the glasscomprises less than about 0.1 mol percent BaO.
 14. A glass according toclaim 1 wherein the glass comprises less than about 0.1 mol percent ofalkali metal oxides.
 15. A glass according to claim 1 wherein the glasshas a liquidus viscosity greater than about 400,000 poises, a densityless than about 2.45 gram/cm³, and a strain point greater than about650° C.
 16. A glass according to claim 1 wherein the glass has aliquidus viscosity greater than about 600,000 poises, a density lessthan about 2.45 gram/cm³, and a strain point greater than about 650° C.17. A glass according to claim 1 wherein the glass has a liquidusviscosity greater than about 800,000 poises, a density less than about2.45 gram/cm³, and a strain point greater than about 650° C.
 18. In aflat panel display device, the improvement comprising a substratecomprising the glass of claim
 1. 19. The flat panel display device ofclaim 18 wherein the substrate has an average surface roughness lessthan about 0.5 nm.
 20. The flat panel display device of claim 18 whereinthe substrate has an average internal stress less than about 150 psi.